The present invention relates in general to substrate manufacturing technologies and in particular to methods and apparatus for igniting a low pressure plasma.
In the processing of a substrate, e.g., a semiconductor wafer, MEMS device, or a glass panel such as one used in flat panel display manufacturing, plasma is often employed. As part of the processing of a substrate (chemical vapor deposition, plasma enhanced chemical vapor deposition, physical vapor deposition, etch, etc.) for example, the substrate is divided into a plurality of dies, or rectangular areas, each of which will become an integrated circuit. The substrate is then processed in a series of steps in which materials are selectively removed (etching) and deposited (deposition) in order to form electrical components thereon.
As device dimensions shrink and more advanced materials are used, the requirements for substantially stable process conditions become even more stringent in order to maintain a uniform etch rate, reduce substrate contamination, etc. This is further aggravated by escalating requirements for high circuit density on substrates that may be difficult to satisfy using current plasma processing technologies where sub-micron via contacts and trenches has high aspect ratios.
In general, there are three types of etch processes: pure chemical etch, pure physical etch, and reactive ion etch. Pure chemical etching generally involves no physical bombardment, but rather a chemical interaction with materials on the substrate. Pure ion etching, often called sputtering, usually uses a plasma ionized inert gas, such as Argon, to dislodge material from the substrate. Etching that combines both chemical and ion processes is often called reactive ion etch (RIE), or ion assist etch.
In these and other types of plasma processes, it may be difficult to ignite a plasma in pressure ranges often required by many plasma processing recipes. In general, when RF power is applied to a set of electrodes, a varying electric field is established between them. If the RF power is sufficiently high, a free electron can be accelerated by the varying electric field until it gains enough energy to collide with an atom or molecule inside the chamber to generate an ion and another free electron. Because of the cascading of the ionization collisions, the entire plasma chamber soon fills with electrons and ions (e.g., plasma). In the plasma, some electrons are continually lost and consumed by collisions with the electrodes, the plasma chamber wall, and also by recombination collisions between electrons and ions and by attachment to neutral species. Hence, the ionization rate of a plasma chamber is mainly determined by the electron energy, which is in turn controlled by the applied power.
Depending on many additional factors including plasma gas chemistry, electrode materials, plasma chamber dimensions, method of RF delivery (e.g., inductively coupled, capacitively coupled, etc.), frequency of electrical excitation, etc., it may be difficult to ignite and sustain a plasma, if the gas pressure is relatively low (e.g., <100 mT in capacitive discharges, etc.). That is, below a critical strike pressure, the plasma may not be self-sustained ignited since the generation rate of electrons caused by ionization collisions may be below the loss rate of electrons. This may be true even in the situation where a previously established plasma can be transitioned to pressures below the critical strike pressure, without the plasma extinguishing.
One solution may be to sustain the plasma by temporarily increasing the gas pressure in order to also increase the plasma gas density and hence the number of collisions with atoms or molecules. That is, increase the generation rate of electrons above the loss rate of electrons. Once the plasma is ignited and stabilized at the higher pressure, the plasma chamber is transitioned to a lower target pressure in order to process the substrate. Target refers to acceptable value ranges in the plasma processing recipe (e.g., target pressure, target power, target gas flow, etc.) However, exposing the substrate to a higher pressure plasma (a non-steady state condition), even if momentarily, may introduce undesirable results on a particular substrate, or unacceptable quality fluctuations between substrates.
Another solution may be to operate the plasma chamber at a higher frequency. In general, higher frequencies tend to more effectively produce the plasma density, due to the more efficient electron heating mechanisms, than when excited by a lower frequency signal at a similar power level. However, increasing the plasma processing frequency may also be problematic. For example, a higher frequency may cause poorer uniformity due to electromagnetic standing wave and skin effects (M. A. Lieberman et al, Standing wave and skin effects in large-area, high frequency capacitive discharges, Plasma Sources Sci. Technol. 11 (2002) 283-293). Other problems may include causing a shift in plasma chemistry and ion/radical ratio because of modified electron temperature, and in difficulty matching tool-to-tool performance due to increased sensitivity to stray capacitance and inductance in the RF delivery and ground return system.
Another solution may be to combine, higher pressure with a different gas flow ratio that is optimized for igniting the plasma, during a short strike step, after which the plasma chamber is transitioned to a lower operational target pressure and appropriate target gas flow ratio in order to process the substrate. However, as before, substantially deviating from the process recipe may introduce undesirable results on a particular substrate, or unacceptable quality fluctuations between substrates.
Yet another well-known method to reduce the critical strike pressure on a narrow gap capacitive discharge system is to increase the electrode gap. In general, mean free path is defined as the average distance a particle can travel before it collides with another particle. Subsequently, increasing the electrode gap may also increases the mean free path, stabilizing a low pressure plasma (e.g., <100 mT, etc.) by increasing the ionization rate of a plasma chamber. However, increasing the gap size may also be problematic. For example, a larger gap may reduce etch rates due to lower power density, increase chamber condition memory effects due to longer gas residence time, and decrease process uniformity when using zoned gas introduction.
For example, dielectric etch reactors often use the combination of capacitively coupled RF excitation sources and relatively narrow electrode to electrode gap spacings. The Lam Research Exelan™ family, for instance, typically uses 1.3 to 2.5 cm gaps compared to 20 to 30 cm substrate diameters. This combination capacitive coupling and narrow gap tends to result in fairly high critical strike pressures which often exceed the desired processing pressure.
Referring now to FIG. 1, a simplified diagram of an inductively coupled plasma processing system is shown. Generally, an appropriate set of gases at a particular pressure and mixed in a particular way may be flowed from gas distribution system 122 into plasma chamber 102 having plasma chamber walls 117. These plasma processing gases may be subsequently ionized at a particular set of RF power settings at or in a region near injector 109 to form a plasma 110 in order to process (e.g., etch or deposit) exposed areas of substrate 114, such as a semiconductor substrate or a glass pane, positioned with edge ring 115 on an electrostatic chuck 116.
A first RF generator 134 generates the plasma as well as controls the plasma density, while a second RF generator 138 generates bias RF, commonly used to control the DC bias and the ion bombardment energy. Further coupled to source RF generator 134 is matching network 136a, and to bias RF generator 138 is matching network 136b, that attempt to match the impedances of the RF power sources to that of plasma 110. Furthermore, vacuum system 113, including a valve 112 and a set of pumps 111, is commonly used to evacuate the ambient atmosphere from plasma chamber 102 in order to achieve the required pressure to sustain plasma 110 and/or to remove process byproducts.
Referring now to FIG. 2, a simplified diagram of a capacitively coupled plasma processing system is shown. Generally, capacitively coupled plasma processing systems may be configured with a single or with multiple separate RF power sources. Source RF, generated by source RF generator 234, is commonly used to generate the plasma as well as control the plasma density via capacitively coupling. Bias RF, generated by bias RF generator 238, is commonly used to control the DC bias and the ion bombardment energy. Further coupled to source RF generator 234 and bias RF generator 238 is matching network 236, which attempts to match the impedance of the RF power sources to that of plasma 220. Other forms of capacitive reactors have the RF power sources and match networks connected to the top electrode 204. In addition there are multi-anode systems such as a triode that also follow similar RF and electrode arrangements.
Generally, an appropriate set of gases at a particular pressure, and mixed in a particular way, is flowed through an inlet in a top electrode 204 from gas distribution system 222 into plasma chamber 202 having plasma chamber walls 217. These plasma processing gases may be subsequently ionized at a particular set of RF power settings to form a plasma 220, in order to process (e.g., etch or deposit) exposed areas of substrate 214, such as a semiconductor substrate or a glass pane, positioned with edge ring 215 on an electrostatic chuck 216, which also serves as an electrode. Furthermore, vacuum system 213, including a valve 212 and a set of pumps 211, is commonly used to evacuate the ambient atmosphere from plasma chamber 202 in order to achieve the required pressure to sustain plasma 220.
In view of the foregoing, there are desired methods and apparatus for igniting a low pressure plasma.